1. Field of the Invention
The present invention relates generally to surgical devices, and more specifically to nondistensible balloon catheters.
2. Description of Prior Art
Balloon catheters are used to dilate or occlude various body conduits, cavities and openings such as blood vessels and the urethra. This is normally accomplished with a catheter formed from an elongate cannula and an inflatable balloon disposed circumferentially of the cannula near the distal end of the catheter.
In accordance with a typical procedure, the catheter is provided with the balloon in a deflated or otherwise low profile state. With this configuration, the catheter is introduced into the body conduit and positioned with the balloon in the low profile state at the point of desired dilatation. At this point the balloon is inflated or otherwise expanded to a high profile state thereby radially stretching the walls of the conduit.
This procedure is particularly applicable in the case of blood vessels which are occluded by plaque. In such a procedure, the balloon in its low profile state is positioned in proximity to the plaque. At this point the balloon is inflated to press the plaque radially against the walls of the blood vessel and thereby dilate or open the vessel to blood flow.
The procedure is also applicable for dilating large body conduits such as the gastrointestinal tract or the prostatic urethra. In the latter case, restrictions are common to men of older age where the prostate, which encircles the urethra, tends to grow inwardly thereby restricting the urethra passage. With the balloon in its low profile state, the catheter can be inserted into the urethra until the balloon is positioned in proximity to the restriction. At this point the balloon can be inflated to dilate the urethra and thereby increase the flow capacity of this conduit.
These dilatation catheters are typically characterized by nondistensible balloons which are formed from materials such as polyethylene that are relatively inflexible and therefore do not expand or distend significantly beyond a known dimension. This characteristic of non-distensibility is of particular advantage in order to insure that the vessel or conduit is not injured by overextension. Unfortunately, the relatively inflexible materials which produce the nondistensible characteristics, tend to inhibit the ability of the balloon to be rolled, compressed, collapsed, deflated or otherwise formed into a low profile state. Such a state is of particular interest in order that the catheter can be easily inserted into the vessel or conduit.
Nondistensible balloons are typically formed circumferentially and co-axially on the cannula. The balloons are characterized by two cylindrical end regions which are attached to the cannula, a cylindrical central region which is spaced from the cannula when the balloon is inflated, and a pair of conical transition regions each extending from an associated end region outwardly to the central region.
With this configuration, the nondistensible balloons of the past have been blow molded from materials which are commonly provided in the configuration of a tube having a substantially constant wall thickness. In the blow molding process, the walls of the tube are expanded against the inner surface of a mold which is provided with a shape desired for the balloon. Ends of the balloon remain at the initial diameter and thickness of the tube, while the central region of the balloon expands to the maximum diameter and minimum wall thickness of the balloon. It is these walls in the central region, which may have a thickness reduced by a factor as much as 100, that dictate the strength of the balloon. Between the central region and each of the end regions of the balloon, one of the transition zones is characterized by a wall thickness which varies from the original thickness of the wall of the tube to the reduced thickness of the wall of the central region.
When these nondistensible balloon catheters are initially inserted, it is of particular importance that the balloon be rolled on the catheter tube to the smallest diameter in order to provide the catheter with the lowest profile possible. The rolling of the balloon has not been a problem in the end regions. Even though end walls contain the thickest wall section, they are fixed in close proximity to the cannula and therefore maintain a low profile even when the balloon is inflated. Similarly, rolling the balloon on the cannula has not been a problem in the central region. Even though the walls in this region are disposed at the highest radial distance from the cannula, the very thin walls in this region are adaptable to being rolled into close compliance with the cannula. However, in the transition regions of the balloon, both the thickness of the wall and the radial displacement of the wall tend to create a problem. Although the transition wall is thinner than that in the end region, it is disposed at a greater radial distance than the end wall. And although this radial distance is less than that of the central region, the wall thickness is greater and therefore more rigid than the central region.
When a nondistensible balloon of the past has been rolled onto a cannula, it has resulted in enlarged sections at each transition region, giving the rolled balloon the appearance of a dog bone. These enlarged transition sections typically have diameters as much as 50% greater than those associated with either the central section or the end section of the rolled balloon. Furthermore they tend to form sharp corners which can severely damage the body conduit during both insertion and withdrawal of the catheter.
With these deficiencies of the prior art, it is an object of the present invention to provide a balloon catheter wherein the balloon can be rolled or otherwise compressed onto the catheter tube to a diameter which is substantially constant along the entire length of the rolled balloon.